Saturday, December 29, 2007

Converting Zone Rating to Something Useful

In Part 1 of my 2007 fielding analysis, I ranked the Tigers on range factor and zone rating.Range factor is not so useful now that more sophisticated measures are available.Zone rating is still regarded as one of the better measures and it’s also very accessible throughout the year. In this post, I will explain how Zone Rating can be translated into something more useful than just a percentage. The Table of Contents for the entire fielding series is shown below:

It would be nice if we could change ZR from a percentage to something like plays made above average or runs saved above average. We can not do this precisely without the actual Balls In Zone data from STATS but we can get estimates. Chris Dial, who writes for Baseball Think Factory and has access to some of the data used for calculating ZR, has developed a method for obtaining these kinds of estimates.

Dial takes the average opportunities at each position and the player's zone rating to calculate the number of plays the player made and compares that to the number of plays that a player with league average ZR would have made. The result is Plays Made Above Average (PMAA). He also estimates the approximate run value of a ball in play at each position. His run value methodology is found here. You will notice that, although it varies by position, an extra play made is worth about 0.8 runs on average. Tom Tango explains it more intuitively on his blog. From that, he determines the runs saved a player aggregates above or below the average player at his position (RSAA).

The Table below shows the ZR, PMAA and RSAA for Tigers players in 2007 (those who were with the team in 2007 and those who have been acquired this off-season). PMAA/150 is PMAA prorated to 150 games played. RSAA/150 = RSAA prorated to 150 games played. PMAA/150 and RSAA/150 allow us to better compare players with different numbers of games played. The final column shows how each player ranks on RSAA/150. This may be slightly different from the ZR ranks due to the games played adjustment.

I'll use Jacque Jones as an example. Jones played 645 innings and had a Zone Rating of .904 as a center fielder in 2007. Given his playing time, he made 5 more plays than the average center fielder. This comes out to 4 runs saved above the average center fielder. If we assume the same level of performance over 150 games, Jones would have made 11 plays above average and saved 9 runs above average.

The leading tigers in runs saved per 150 games in 2007 were: Brandon Inge (12), Curtis Granderson (10), Magglio Ordonez (10) and Sean Casey (8). The only Tiger who was below average was Carlos Guillen who had a RSAA/150 of -7. New acquisition Jacques Jones finished 9 runs above average. However, Miguel Cabrera (-16) and Edgar Renteria (-9) were below average. Based on this statistic, it appears that the Tigers did not help their defense during the off-season.

The Replacement Level Yankee Weblog now includes a zone rating database that has Zone Ratings along with conversions to runs saved above average for all players from 1987-2007. Just go to the left sidebar and you'll see a link to the database.

1 comment:

I'm a big fan of zone rating. It's one of those stats where, when I look at it at the end of the season I think, "yeah, that seems right." I think it's hard to tell just by these numbers just how much we might miss Inge next year. Obviously the strikeouts stand out in the mind more than the defensive stops. While Renteria isn't GREAT anymore, he's still smoother out there than Guillen.